1. Field of the Invention
The present invention relates to a novel lighting circuit for a discharge lamp, which cuts off power supply to the discharge lamp when the input voltage to a DC power supply section drops, and which performs such power control that even when the discharge lamp is turned off due to the dropping of the battery voltages, power is supplied to the discharge lamp to turn it on when there is a sign of the restoration of the dropped input voltage and does not over-respond to a variation in the input voltage to thereby avoid repeating the periodic status change. This lighting circuit can be adapted for a vehicular discharge lamp, for example.
2. Description of the Related Art
Recently, compact discharge lamps (e.g., metal halide lamps) are receiving greater attention as a light source to take the place of an incandescent lamp. To adapt a discharge lamp to the light source for a vehicular lamp, it is necessary to take some measures to protect the lighting circuit against a change in the input voltage.
This protective measure should be taken to prevent the lighting status of a discharge lamp from becoming unstable or to prevent the malfunction or the like of the lighting circuit in response to a variation in the input voltage. For example, there has been proposed a lighting circuit which is equipped with a circuit for detecting the input voltage, causes a power cutoff circuit to inhibit power supply to a discharge lamp when the input voltage drops lower than a threshold value and maintains the power cutoff state unless the ignition switch is switched on again.
This proposed circuit however keeps cutting off power supply to the discharge lamp even if the input voltage, once dropped below the threshold value, has returned to a level higher than the threshold value. In other words, even though the input voltage is restored to the level enough to keep the lighting of a discharge lamp, this circuit cannot ignite the discharge lamp unless a user switches on the ignition switch again.
When the input voltage is restored to or higher than the threshold value, the inhibition of power supply to the discharge lamp should be released to turn on the discharge lamp.
It is however necessary to consider the voltage drop of the input voltage to the lighting circuit caused by the wiring or the like. When the wires to connect the power supply to the lighting circuit are long as in the case of a lighting circuit for a discharge lamp as a vehicular lamp, for example, the voltage drop becomes larger accordingly. If the threshold value is set to a constant value regardless of the status of the discharge lamp, the power supply to the discharge lamp and the power cutoff may be repeated in short cycles due to a change in the input voltage.
FIG. 10 is a conceptual diagram for explaining how such repetition occurs, showing the input voltage (indicated by "B") on the horizontal scale. FIG. 10 illustrates the operational statuses on the lighting circuit separated into four modes by a solid line a passing a point Bs or the threshold value and perpendicular to the horizontal scale and a broken line b parallel to the horizontal scale.
The modes (1) and (2) on the left-hand side of the solid line a are distinguished from the modes (3) and (4) on the right-hand side of the solid line a, depending on if the input voltage B is greater than the threshold value, while the modes (1) and (4) above the broken line b are distinguished from the modes (2) and (3) below the broken line b depending on whether or not the discharge lamp is turned on.
When the input voltage B drops during the lighting of the discharge lamp shown by the mode (4), the operational state enters the mode (1). When the discharge lamp is turned off then, the operational state enters the mode (2). The mode (3) is the state where the discharge lamp is turned off, causing the current flowing into the lighting circuit to become zero, and the input voltage B is restored. When the discharge lamp is lighted thereafter, the operational state proceeds to the mode (4).
A variation in the input voltage B caused by the voltage drop may cause a short cycle of the mode change from the mode (4), to the mode (1), to the mode (2), to the mode (3) and back to the mode (4) in the case where the lighting of the discharge lamp fails due to the reduced input voltage B or where the ignition swatch is activated again to turn on the discharge lamp when the input voltage B has dropped. Such a phenomenon, a kind of chattering, would interfere with the control stability.